Broadband access is increasingly moving to fibre technology such as passive optical networks (PONs), for example GPONs (gigabit passive optical networks) or EPONs (Ethernet passive optical network). PONs can deliver voice, video and data services among multiple network nodes, often referred to as optical network terminals (ONTs) or optical network units (ONUs), using a shared optical fibre. Passive optical splitters and combiners enable multiple ONTs to share the same optical fibre link. For example, downstream information carried by the shared optical fibre link may be optically split for transmission to multiple ONTs via individual optical fibres. Likewise, upstream information received from individual ONTs via individual optical fibres may be optically combined for transmission via the shared optical fibre link. An ONT, sometimes referred to as a subscriber premises node, may be connected to one or more subscriber devices, such as televisions, set-top boxes, telephones, computers, or network appliances, which utilize voice, video and data services delivered via the PON.
A PON typically includes a PON interface, sometimes referred to as an optical line termination (OLT), which may have multiple PON interface modules. The PON interface modules serve respective optical fibre links. An OLT provides an interface for downstream transmission and upstream reception of information over a shared optical fibre link that serves a group of ONTs. The OLT may be coupled to an optical splitter/combiner via the shared optical fibre link. A PON is inherently a downstream-multicast medium. Each downstream communication on the shared optical fibre link can be received by every ONT served by that link. ONTs may identify selected packets or frames on the optical fibre link based on addressing information included within the packets or frames. In addition, individual ONTs transmit upstream packets to the OLT via the shared optical fibre link.
The addressing information may be identified using the Address Resolution Protocol (ARP), which is a networking protocol for determining a network node's Link Layer or hardware address when only its Internet Layer (IP) or Network Layer address is known. In the case of IPv4 Ethernet networks, ARP is used to translate IPv4 addresses (OSI Layer 3) into Ethernet Media Access Control (MAC) addresses. In the next generation Internet Protocol, IPv6, ARP's functionality is provided by the Neighbour Discovery Protocol (NDP). It will be appreciated that ARP is not specifically associated with PONs, but is often used in conjunction with PONs when the electrical side of an ONT or OLT is connected to an Ethernet network.
Schemes have been proposed for these technologies to carry out automatic protection switching to guard against failure of an optical fibre in an optical distribution network (ODN). Each ONT/ONU is connected to at least two optical fibres, which may be routed to the same OLT or different OLTs. A protection switch determines whether there is an optical fibre failure in a fibre link in the ODN upstream of an ONT and, if so, switches to a different link or ODN. Such schemes currently specify how the automatic protection switching itself is carried out, but do not make any provision as to how traffic flow through the network should be updated. In order for an approach that can operate in an Ethernet network built with multiple vendor's equipment, a method is needed that will work with most standard equipment.
An additional problem is that various protocols have been defined for a client device to send messages, but they are rarely reliable and differ a lot between client devices and their operative systems. For example, Gratuitous ARP messages are used when the client device's network interface is being connected. Gratuitous ARP messages from the client device cannot be used for the purpose of updating during a protection switch, since the link towards the client device is not affected, and therefore not noticed by the client devices. When protection switching takes place at an ONT, the northbound link is switched, but the southbound link towards the client device is maintained. This means that client devices will not notice the switch and will not send any messages.
The problems can be understood with reference to FIG. 1, which is a schematic illustration of a typical architecture of a network 100 for delivering data from an application server 101 to a client device 102. The application server 101 is connected via layer 2 (Ethernet) network switches 103, 104, 105 to two OLTs 106, 107. Each OLT 106, 107 is connected by an optical fibre 108, 109 to a splitter 110, 111 in a respective optical distribution network (ODN) 112, 113. An ONT/ONU 114 is connected by optical fibres 115, 116 to both splitters 110, 111. The ONT/ONU 114 is also connected to the client device 102. It will be appreciated that, normally, many ONT/ONUs will be connected to the ODNs 112, 113, and that each ONT/ONU may be connected to one or more than one client device. In this example a single ONT/ONU 114 and client device 102 is shown for clarity.
The ONT/ONU 114 is thus connected to two different ODNs 112, 113. Initially the ONT/ONU is only actively forwarding client device traffic through one of the OLTs 106 and a “primary” ODN 112.
If a fibre 108, 115 breaks, or for some reason the 106 OLT is not properly working, the ONT/ONU 114 is able to detect this. The ONT/ONU then decides to do a switch over to a secondary ODN 113 and start forwarding traffic through its corresponding OLT 107, as set out in the defined protection schemes.
The problem that arises when the automatic protection switching happens is that the switches 103, 104, 105 in the Ethernet Network (L2 Network) are unaware about this. Any traffic which is ongoing is still being forwarded through the ports of the switches 103, 104 where the MAC address of the client device 102 was first learned. It is possible to let the MAC address entries in the switches time out so they can be re-learned, but this is very time consuming. An alternative solution relies on the fact that the client device 102 may be sending traffic, and these packets would therefore update the learned entries in the Ethernet network 103, 104, 105. In traffic scenarios where traffic is uni-directional during a longer period this could become a issue, since the time elapsed before the client device sends any traffic could be significant. This could be seen in UDP traffic, where there is no acknowledgement of packets.
Both of the approaches described above are too slow and will in most cases cause traffic interruption and packet loss. With some applications and/or protocols they may also loss of connectivity because of the long switch over time and re-learning time.